Chimera is our next generation molecular graphics application whose main design goals are extensibility and ease of use. Chimera is being written in Python with C++ extensions and uses commonly accepted application programmer interface (API) specifications such as the ``Tk'' (TCL toolkit) for it's graphical user interface and OpenGL for three- dimensional graphics primitives. In addition, Chimera also uses the locally developed Object Technology Framework object class library for manipulating molecular data. Because program extensibility is a major design goal for this project, we chose to use a complete programming language (Python) as the Chimera command language. This will allow users to create complex Chimera command ``scripts'' (e.g. iterative loops and conditional execution), which in turn will allow sophisticaed operations to be perfromed on multiple molecular models. Python is an interpreted language with an extensive library that includes interfaces to Tk, which means users will be able to easily create their own custom graphical user interface (GUI) elements (such as menus and dialog boxes) as part of their extensions. Chimera itself is implemented with a small set of core functionality, including graphical display, Protein Data Bank (PDB) input, and basic user interface elements (menu bars, tool bar, command line, reply window and status line). More advanced features are constructed on top of the core using Python extension modules. This results in a program architecture in which new functionality is easily added when needed. In fact, we have used this basic extension mechanism to much of the standard functionality present in Chimera, since it is both fast and easy to program this way, and it insures that the small set of core functions we do provide are adequate to build whatever extensions a user may desire. Recent Chimera development has been focused on removing its previous dependence on Motif and OpenInventor. (Motif is a GUI window manager that is popular on UNIX-based workstations but is not commonly used on other platforms such as Microsoft Windows or Apple Macintosh. OpenInventor is an object-oriented interface to the popular OpenGL graphics package, but only Silicon Graphics currently has OpenInventor available for their workstations.) Removing Chimera's dependence on Motif and OpenInventor increases the portability of Chimera significantly, particularly with respect to Windows NT. A number of user interface elements have also recently been added (e.g., mouse panel, lenses, and selection panel). Two new extensions to Chimera (a Multiple Sequence Format (MSF) file viewer and a stru cture alignment viewer) are also under development and were demonstrated at our Advisory Committee meeting on March 16, 1998; they were well received.